Modern motor vehicles are supplied with driver's side airbag modules. Airbag modules are most commonly mounted in the center of the steering wheel, the location where a horn-activating switch has traditionally been mounted. When driver's side airbags were first introduced, the horn-activating switch was moved from the center to another location on the steering wheel to make room for the airbag. The horn-activating switches were often mounted on the steering wheel spokes or rim. However, many drivers preferred the traditional placement of the horn-activating switch.
Eventually, the horn-activating switch was adapted for mounting on the underside of the airbag module cover, allowing the horn-activating switch to be placed in its traditional position. Such horn-activating switches are pressure sensitive switches that detect when the user is applying force to the cover in effort to sound the horn. However, placing the pressure sensitive switches beneath the horn cover subjects the switch to the expansion and contraction (shrinking) of the cover. As the temperature within the vehicle decreases, the airbag module cover shrinks, causing a force to be applied to the pressure sensitive switch. This force may increase to the level required to sound the horn, resulting in an unexpected sounding of the horn. In addition, changes in the stiffness of the cover due to changes in temperature can cause a variation in the pressure required to activate the horn.
U.S. Pat. No. 5,489,806 describes an airbag horn switch with temperature compensation. A temperature sensitive device, such as a thermistor, is attached to the airbag cover along with the pressure sensitive horn switch. The change in characteristics of the temperature sensor is used to compensate for temperature variations by adjusting the magnitude of the force required to activate the horn. However, the use of two sensors, a temperature sensor and a pressure sensor, increases the complexity and cost of the circuit.